One is server, the other is the client (always the case with Bluetooth).

When does the pairing happen? I can think of a few times:

Use case 1 - 
 * Swapper decides to use bluetooth to send apps to others.
 * Selects "Use bluetooth instead" on the "join wifi" screen.
 * Starts a bluetooth server 
   + Make itself discoverable
   + Opens a bluetooth server socket
   + Waits for incoming client connections.

 * Swapee opens swap workflow
 * Selects the bluetooth option
 * Is asked to pair with nearby bluetooth devices, using the F-Droid UUID to make sure it doesn't connect to, e.g. bluetooth headphones.
 * Stays connected in the background
 * Adds the repo as per usual (with a url such as bluetooth://device-mac-address)
 * When repo updates, it uses the open connection to get data
 * If the connection has closed, attempts to reconnect
 * Same when downloading files